1. Field of the Invention
The present invention relates to an electrophoretic display device, and more particularly to an electrophoretic display device having an improved dispersion medium.
2. Description of the Related Art
Electrophoretic display devices, which use electrophoresis, are one type of non-emitting display device. Electrophoresis is a phenomonon whereby the Coulomb force causes microparticles (e.g., electrophoretic particles) charged naturally by dispersion to migrate when an electrical field is applied to a fluid dispersion medium in which the microparticles are contained.
The basic structure of an electrophoretic display includes first and second electrodes disposed in opposition with a specific space therebetween. This space is filled with an electrophoretic suspension. When a potential difference is applied between the electrodes, the electrophoretic particles are pulled to one or the other of the electrodes depending on the direction of the electrical field. If the dispersion medium is dyed with dye and pigment particles are used for the electrophoretic particles, the color of the pigment particles or the color of the dye can be seen from the viewing side of the display, that is, the side where the transparent substrate and transparent electrodes are disposed. An electrophoretic display thus comprised can therefore be used to display images by forming the electrodes in a desired pixel pattern and controlling the voltage applied to the pixel electrodes.
Dodecylbenzene is described as the dispersion medium in this type of electrophoretic display in, for example, a paper (in Japanese) entitled xe2x80x9cAn electrophoretic display using microcapsulesxe2x80x9d (Japan Hardcopy 1999 collection of papers, pp. 237 to 240) written by an engineer at NOK corp. This paper describes placing the electrophoretic suspension inside the microcapsules instead of directly filling the dispersion medium between the electrodes.
Japanese Patent Publication (kokoku) S49-32038 is directed to an electrophoretic display having cells disposed between the electrodes, dividing the electrophoretic suspension into pixel units. Migration of the electrophoretic particles is restricted to within their designated cells. Using such cells makes it possible to increase the image resolution of the electrophoretic display and easily develop a color display.
Using dodecylbenzene as the dispersion medium of the electrophoretic display, however, causes certain problems relating to reliability and response. Dodecylbenzene attacks the sealants used to inject and seal the suspension inside the display, and there is therefore a danger of the suspension being incompletely sealed in the display.
An object of the present invention is therefore to improve the reliability and response of the electrophoretic display device by employing an improved dispersion medium.
To achieve this object, the present invention provides a dispersion medium in which electrophoretic particles are dispersed, wherein the dispersion medium includes at least one organic compound having a structural formula containing at least two rings. An electrophoretic display device including such a dispersion medium is also provided.
The at least one organic compound preferably comprises at least one material defined by any of the following structural formulae (1) to (4) shown below:
X1-A1-B1-A2-Y1xe2x80x83xe2x80x83(1)
xe2x80x83X3-C1xe2x80x83xe2x80x83(3)
X4-C2-B4-A6-Y3xe2x80x83xe2x80x83(4)
The content of such organic compound(s) in the dispersion medium is preferably 30% by weight or more; more preferably, the content of such organic compound(s) is 80% by weight or more; and even more preferably, such organic compound(s) constitute 100% by weight of the dispersion medium.
In the formulas (1) to (4), each of A1 to A6 is at least one of a benzene ring, a cyclohexane ring, a dioxane ring, a pyrimidine ring, a furan ring, a tetrahydrofuran ring, a dithiane ring, a piperidine ring, a pyrazine ring, a morpholine ring, a cyclopentane ring, a cyclobutane ring, a pyridine ring, a thiophene ring, a pyrrole ring, or a cyclohexene ring.
In the formulas (1) to (4), each of X1 to X4, Y1 to Y3, and Z is at least one substituent selected from the group consisting of hydrogen, fluorine, chlorine, bromine, iodine, cyano, nitro, amino, hydroxyl, alkanoyloxy, alkoxycarbonyl, carboxy, trifluoromethyl, trifluoromethoxy, alkoxymethylene, alkyl, alkoxy, alkenyl, sulfonate, thioalkyl, monoalkylamino, dialkylamino, and trialkylamino.
In the formulas (1) to (4), each of B1 to B4 is either a single bond or comprises one of the following: COO, OCO, CHxe2x95x90N, Nxe2x95x90CH, CHxe2x95x90CH, (CH2)nO, O(CH2)n, (CH2)n, O(CH2)nO, CHxe2x95x90CHxe2x80x94COO, OCOxe2x80x94CHxe2x95x90CH, O, CO, NHCO, CONH, Nxe2x95x90N, N2O, Cxe2x89xa1C, SO, SO2, or NH, where n is an integer from 1 to 10.
In the formulas (1) to (4), each of C1 and C2 comprises naphthalene, tetrahydronaphthalene, decahydronaphthalene, indene, quinoline, coumarin, azulene, fluorene, or dihydrophenanthrene.
Formulas (5) to (32) shown in Appendix A are more specific examples of compounds having any of the above structural formulas (1) to (4), where R1 to R33, X, and Y are as follows.
Each of R1 to R33 is an alkyl, alkoxy, alkoxymethelene, halogen elements, hydrogen, alkenyl, cyano, nitro, amino, hydroxyl, alkanoyloxy, alkoxycarbonyl, trifluoromethyl, carboxy, monoalkylamino, dialkylamino, trialkylamino sulfonate, thioalkyl, trifluoromethoxy, acyl, acyloxy, or formyl.
Each of X and Y is either a single bond or comprises one of the following: COO, OCO, CHxe2x95x90N, Nxe2x95x90CH, CHxe2x95x90CH, (CH2)nO, O(CH2)n, (CH2)n, O(CH2)nO, CHxe2x95x90CHxe2x80x94COO, OCOxe2x80x94CHxe2x95x90CH, O, CO, NHCO, CONH, Nxe2x95x90N, N2O, Cxe2x89xa1C, SO, SO2, or NH, where n is an integer from 1 to 10.
Of these organic compounds, ones with small optical anistropy are preferably used. Scattering, birefringence, and optical rotatory polarization depending on the orientation of the molecules occur easily when an organic compound with large optical anistropy is used as the dispersion medium, and such compounds thus contribute to a drop in light uniformity. By using materials with small optical anistropy, and more preferably optically isotropic materials, highly uniform light can be obtained from each pixel of the electrophoretic display device, and a wide viewing angle can be achieved.
Examples of the organic compounds with small optical anisotropy denoted by any of structural formulae (1) to (4) above include cyclohexyl cyclohexane, phenylcyclohexane, phenylethylcyclohexane, phenylcyclohexanecarboxylate, 4-phenyl-1,3-dioxane, and 2-phenylethyl-1,3-dioxane types.
The present invention is ideally suited to an electrophoretic display device wherein the space between opposing electrodes is divided into cells representing pixel units, and migration of each electrophoretic particle is restricted to within the particular cell in which it is contained. Each cell is filled with a suspension having electrophoretic particles dispersed in a dispersion medium. Units of three adjacent cells form a group, and the three cells of each group are respectively filled with a suspension corresponding to red (R), green (G), and blue (B).